Fixed three errors found while testing the #79 fix:
- The routing of the skb needed to happen before the initialization of most of the fragment's fields, because the latter can generate ICMP errors.
- I just realized that if the BIB or session databases are too big, Jool will flood dmesg with lots of identical error messages. Fixed, but did introduce issue #88.
- When Jool didn't recognize the layer-4 protocol of a IPv6 packet, the wrong ICMP error was being generated.